Defective Interfering (DI) particles of Vesicular Stomatitis Virus (VSV), with deletions ranging from 85 percent to 50 percent were investigated. Annealing experiments with the complementary viral mRNAs suggested that the conserved sequences in DI particle RNAs are at or near the 5 feet terminus while the 3 feet termini were variable. Extracistronic sequences, obtained from the single stranded part of DI particle RNAs after annealing with mRNA are under investigation. A comparison of these sequences to the corresponding viral nucleotide sequences are in progress, in order to determine whether any modification or duplication of the replicase promoter region has taken place. Such a change would explain the mechanism of autointerference. Similar studies are also carried out with unique heterotypically inferfering DI particle RNA in order to elucidate changes in the promoter site which would explain the breakdown of specificity. DI particles from distantly related serotypes are also being investigated in the search for general features of auto-interference in rhabdoviruses.